Yes, you can use IMAP and POP3 at the same time. We mainly use IMAP, but POP3 allows cellphones and stuff to easily access the Inbox (and nothing else). There are some issues with reading mail in the IMAP client causing it to remark the messages as new to the POP3 client or something, but I've never had a problem with it, probably because I never have any of the POP3 clients remove mail from the server.
If you switch to IMAP, you can also use procmail to filter mail (including spam) into subfolders too. I have Razor2 and SpamAssassin running on our 5.5U5, and it works great.